By default SQL Server will auto grow your databases by 1 MB for your data file and 10% for your log files. There have been numerous discussions about the usefulness of this default value, but at the end of the day you should really understand how this will effect your database.
In the world of SharePoint it’s common for users to upload multiple mega-byte files, which will be stored in the content database and would cause the database file to grow and could potentially cause performance issues.
Another SharePoint scenario that needs consideration is the SSP databases, the indexing of content performs a large number of write operations of the metadata associated with the crawled content. It might be wise to monitor the performance of these databases to determine if changing the auto growth rate would help.
If you came to the conclusion that you did want to change the default options for all new databases, then you can simply change the default values of the model database, you can do this by right clicking on the model database and selecting the button next to the file you wish to change:
Like all performance related tips, it’s best if you conduct some of your own analysis.